Regex Question

[James Stroud]

Bill Mill wrote:
> Hello all,
> 
> I've got a test script:
> 
> ==== start python code =====
> 
> tests2 = ["item1: alpha; item2: beta. item3 - gamma--",
> "item1: alpha; item3 - gamma--"]
> 
> def test_re(regex):
>    r = re.compile(regex, re.MULTILINE)
>    for test in tests2:
>        res = r.search(test)
>        if res:
>            print res.groups()
>        else:
>            print "Failed"
> 
> ==== end python code ====
> 
> And a simple question:
> 
> Why does the first regex that follows successfully grab "beta", while
> the second one doesn't?
> 
> In [131]: test_re(r"(?:item2: (.*?)\.)")
> ('beta',)
> Failed
> 
> In [132]: test_re(r"(?:item2: (.*?)\.)?")
> (None,)
> (None,)
> 
> Shouldn't the '?' greedily grab the group match?
> 
> Thanks
> Bill Mill
> bill.mill at gmail.com

The question-mark matches at zero or one. The first match will be a 
group with nothing in it, which satisfies the zero condition. Perhaps 
you mean "+"?

e.g.

py> import re
py> rgx = re.compile('1?')
py> rgx.search('a1').groups()
(None,)
py> rgx = re.compile('(1)+')
py> rgx.search('a1').groups()

James